Nondestructive testing using electromagnetic radiation is used for a variety of industrial applications and medical applications. X-rays are electromagnetic waves having a wavelength in the range of, for example, about 1 pm to 10 nm (10−12 to 10−8 m). X-rays having a short wavelength (energy of about 2 keV or larger) are called hard X-rays, and X-rays having a long wavelength (energy in the range of about 0.1 keV to about 2 keV) are called soft X-rays.
For example, an absorption contrast method obtains an absorption image of an object by using the difference in transmittance of X-rays passing through the object. Because X-rays easily penetrate through an object, the absorption image is used for internal crack detection of steel materials and for security purposes such as baggage inspection.
On the other hand, an X-ray phase imaging method detects phase shift of X-rays caused by an object. The X-ray phase imaging method is effective for an object made of a material having a small density difference, because absorption contrast of X-rays is not clear for such a material. For example, the X-ray phase imaging method can be used for imaging a phase separation structure made of polymer blends or for medical applications.
Patent Literature 1 describes a very convenient and effective method of X-ray phase imaging that uses a refraction effect due to phase shift of X-rays caused by an object. To be specific, the method uses a microfocus X-ray source and utilizes an effect that the edge of an object is detected in an enhanced manner owing to a refraction effect of X-rays caused by the object when the distance between the object and a detector is large. Because the method uses the refraction effect, it is not necessary to use highly coherent X-rays such as synchrotron radiation, which distinguishes the method from many other X-ray phase imaging methods.
Patent Literature 2 discloses an X-ray imaging apparatus having a mask that blocks X-rays on an edge portion of the pixel of the detector. By setting the mask so that a part of the mask is irradiated with X-rays when an object is not present, a displacement of X-rays due to a refraction effect caused by the object can be detected as a change in intensity.